In treating sewage, it is often necessary to dissolve large quantities of oxygen in the sewage so as to oxygenate it. Presently known methods include The BOC Group PIc's VITOX.RTM. apparatus as described in British Patent Number 1,455,567. This apparatus comprises a venturi device having a plurality of small holes provided around the circumference of the throat for the introduction of oxygen into the liquid passing through the venturi. Oxygen is generally provided from a liquid store and the pressure of the released gas is usually over 6 bar(g) and sufficiently above the 1.8 bar(g) operating pressure of the VITOX.RTM. unit to ensure the oxygen can be introduced into the liquid. Alternatively, one could arrange the venturi such that its operating pressure in the venturi throat is somewhat lower than normal and hence less gas pressure would be required to ensure the oxygen is passed into the liquid. This alternative arrangement is not a preferred one as typically only 80% of the lost liquid pressure is regained by the venturi and, hence, a large amount of energy has to be expended in liquid pumps to provide the extra pressure.
An improvement in such apparatus is provided in accordance with the present invention which pertains to an apparatus for introducing oxygen into a liquid, such as sewage, that is particularly well suited to operation at low gas pressures without increasing liquid pressures thereby reducing energy wastage and improving efficiency.